dep-2.js

// the current target watcher being evaluated.
// this is globally unique because there could be only one
// watcher being evaluated at any time.
Dep.target = null
const targetStack = []

function pushTarget(_target) {
  if (Dep.target) targetStack.push(Dep.target)
  Dep.target = _target
}

function popTarget() {
  Dep.target = targetStack.pop()
}